As active carbon fibers for adsorption and separation, several kind are known. These are, for example, those which are made from regenerated cellulosic fibers, acrylonitrile fibers, phenolic fibers and pitch fibers. Fibrous active carbon has many merits in its shape such as a much greater contact area than particulate active carbon and, thus, higher adsorption and desorption rates. Furthermore, by employing hollow fibers, troublesome steps of adsorption and desorption can be omitted and separation from a fluid becomes possible simply by passing fluid through the hollow fibers making an energy-saving process possible. With reference to hollow active carbon fibers, Japanese Patent Kokai No. 48-87121 discloses a hollow fiber, having the ability to adsorb trace substances in a gas or liquid which is made by forming voids in a carbon material to achieve a void content of 10-80% and a specific surface area of 400 m.sup.2 /g or more. The method for making this hollow fiber comprises crosslinking the skin portion of fiber made from phenol as raw material, extracting the uncrosslinked core portion with a solvent to obtain a hollow fiber, carbonizing this hollow fiber and furthermore making it porous by activation treatment with an oxidizing gas such as water vapor. Therefore, the resulting pores are micropores of 10-20 .ANG. in radius, and the hollow portion of the resulting hollow fiber lack, uniformity and is high in flow resistance and exhibits alow permeation rate.
Japanese Patent Kokai No. 58-91826 discloses a pitch based hollow carbon fiber, but the inner diameter of the hollow portion is small, less than 10 .mu.m, and there are no pores in the membrane wall. Thus a membrane for separation is not intended.
Japanese Patent Kokai Nos. 60-179102 and 60-202703 disclose carbon membranes of multi-layer structure, but the carbon membrane of the former has at least one finely porous dense layer and at least one layer has large pores for the increase of the permeation rate and the orientation coefficient as the whole multi-layer structure is small, 0.7. The carbon membrane of the latter also comprises porous layer having separating ability and porous layer of sponge structure having voids of 5 .mu.m or more in maximum pore diameter for the increase of the permeation rate and is of very brittle membrane structure and cannot be used practically.
Furthermore, Japanese Patent Kokai No. 61-47827 discloses a carbonized hollow fiber from polyvinyl alcohol fiber and according to the disclosure, a dehydrating agent is penetrated into only the surface layer portion and the portion is infusibilized at the carbonization step and the central portion in which the dehydrating agent is not penetrated is molten and removed to make it hollow. In addition the fiber is subjected to an activation treatment with water vapor to make a porous hollow carbon fiber.
Japanese Patent Kokai No. 63-4812 (=EP252339) has proposed a method for making a carbon membrane having pores by which a hollow fiber membrane provided with pores by the extraction method is treated with an aqueous hydrazine solution and then subjected to oxydizing treatment and carbonizing treatment.
However, porous hollow carbon fibers of these conventional techniques mostly have micropores of 1-5 nm and are insufficient in properties such as strength from a practical viewpoint.
The average radius of pores of active carbon fiber and porous hollow carbon fiber of the conventional techniques is small, namely, 1-5 nm and are suitable for adsorption or separation of substances of relatively small molecular weight from the vapor phase, but are not suitable for the adsorption and separation of substances of relatively large molecular weight from vapor or liquid phase as aimed at by the present invention. Furthermore, most of the fibers of the conventional techniques are low in elongation and lack flexibility.